1. Field of the Invention
The present invention relates to an electric motor.
2. Description of the Related Art
Conventional electric motors, which are provided with a first rotor and a second rotor in a concentrically circular arrangement around a rotational axis of the electric motor and which control the phase difference, that is, the relative positions in the circumferential direction of the first rotor and the second rotor in response to the rotational speed of the electric motor, or in response to the speed of rotating magnetic field generated in the stator, are well known (for instance, refer to Japanese Unexamined Patent Application, First Publication No. 2002-204541).
For instance, to control the phase difference of the first rotor and the second rotor in response to the rotational speed of the electric motor, the relative positions in the circumferential direction of the first rotor and the second rotor are changed in this electric motor through members that displace along the radial direction under the action of centrifugal force. For example, to control the phase difference of the first and the second rotors in response to the speed of the rotating magnetic field generated in the stator, a control current is passed through the stator coil with the rotational speed maintained by the inertia of each rotor, and the speed of the rotating magnetic field is changed so as to change the relative positions in the circumferential direction of the first and second rotors.
Incidentally, in the electric motor according to an example of the prior art mentioned above, during the control of the phase difference of the first and second rotors in response to the rotational speed of the electric motor, for example, the problem that arises is that the phase difference of the first and second rotors can be controlled only in the state when centrifugal force corresponding to the rotational speed acts, that is, only in the operating state of the electric motor, and the phase difference cannot be controlled at the correct timing in the stopped state of the electric motor in electric motors. Also, another problem is that when this electric motor is provided in a vehicle as the drive source and when this electric motor is susceptible to external vibrations, it is difficult to control the phase difference of the first and second rotors correctly by the action of centrifugal force only. Furthermore, in this case, since the phase difference is controlled regardless of the fluctuation in the power supply voltage of the power source for the motor, there is a possibility of an abnormality to occur, such as inversion of the magnitude relationship between power supply voltage and back electromotive force of the electric motor.
Also, for instance, when controlling the phase difference between the first and second rotors in response to the speed of rotating magnetic field that occurs in the stator, another problem arises that the speed of the rotating magnetic field is changed, and thus the control process of the electric motor becomes complex.
The present invention was made in view of above circumstances and has an object of providing an electric motor which can extend an operable revolution speed range and a torque range by employing an induced voltage constant which can be easily and appropriately changed while inhibiting a complexity of the electric motor, and which thereby enables increasing operating efficiency and extending an operable range at high efficiency.